Follow-up from "WIP: API endpoints for Group-level Security Dashboard"

The following discussions from !7757 (merged) should be addressed:

• @fcatteau started a discussion: (+16 comments)

  @ayufan The needed indexes for counting vulnerabilities.

• @ayufan started a discussion:

  Follow-up item, to create an issue before merging:

  1. We should refactor VulnerabilityFeedback to move it to Vulnerabilities::Feedback to follow the new code architecture,
  2. Move schema matcher of vulnerability_feedback.json into vulnerabilities/feedback.json.

• @abrandl Estimate the expected amount of data (and growth over time). See https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7757#note_106574518.